Image orthicon tube having specially coated decelerating field electrode



' June 9 1964 D. L. SCHAEFER 3 l IMAGE ORTHICON TUBEYHAVING SPECIALLYCOATED 36916 DECELERATING FIELD ELECTRODE Filed May 17, 1961 FIG.3. 20 f2 El INVENTOR:

%LD L. SCHAEFER, BY M HIS TORNEY.

United States Patent 7 3,136,916 IMAGE ORTHICON TUBE HAVING SPECIALLYCOATED DECELERATING FIELD ELECTRODE Donald L. Schaeter, Scotia, N.Y.,assignor to General Electric Company, a corporation of New York FiledMay 17, 1961, Ser. No. 110,817 4 Claims. .(Cl. 315-) This inventionrelates to cathode ray tubes and more specifically to picture signalgenerating tubes of the kind in which video signals are obtained byscanning with an electron beam a planar electric charge-storing target.More particularly the invention relates to improvements in camera tubesof the image orthicon type wherein the charge storing target is mountedin confronting adjacent relationship with a decelerating field formingforaminate electrode disposed in the path of the target-scanningelectron beam.

As is known to those skilled in the art, the stored charge pattern onthe target of a tube of the image orthicon type is read by the chargeneutralizing etiect of deposition on the target of some of the electronsof the scanning electron beam, and the resulting modulation of beamcurrent is amplified in the electron multiplier portion of the tube towhich the scanning beam is returned.

It has also been recognized that, in image orthicon type tubes having adecelerating field-forming forarninate or mesh electrode disposedopposite the target in the path of the scanning electron beam, secondaryelectrons are emitted from such decelerating field forming meshelect'rode by impingement oh the mesh of primary electrons of thetarget-scanning electron beam. Some of such secondary electrons aredrawn or accelerated to the electron multiplier portion of the tube andthereby result in the generation of spurious signals which deleteriouslyefiect the overall signal to noiseratio of the tube.

Accordingly, a principal object of the present invention is to providean image orthicon tube including a decelerating field forming mesh,hereinafter called field mesh, having improved output signal to noiseratio.

Another object is to provide a field mesh image orthicon having reducedemission of secondary electrons from the field mesh electrode.

Another object is to provide an improved field mesh electrode for animage orthicon of the field mesh type, having minimized secondaryelectron emission capability.

These and other objects of the invention will be apparent from thefollowing description and the accompanying drawing wherein:

FIGURE 1 is a fragmentary View, partially broken away in transversesection, of a portion of an image orthicon type camera tube constructedin accordance with the present invention;

FIGURE 2 is an exploded perspective view of a portion of the structureshown in FIGURE 1;

FIGURE 3 is an enlarged fragmentary sectional view of a portion of thestructure shown in FIGURE 1.

Briefly, according to the present invention I have discovered thatsubstantial reduction of spurious signals resulting from the field meshelectrode of a field mesh type image orthicon can be obtained by makingat least the surface portions of said field mesh electrode facing normalto the scanning electron beam of a material which has a secondaryelectron emission characteristic of less than 1.0. Preferably I securethis low secondary emission characteristic by coating such surfaces ofthe field mesh electrode with chromium.

Alternative to chromium, other low secondary emission characteristicmaterials compatible with the environment may also be employed for suchsurfaces, such as a material from the group consisting of manganese, ti-

tanium, vanadium, and alloys and intermetallic compounds thereof.

Turning to FIGURE 1 of the drawing, the target end portion of the cameratube there shown includes an envelope 2 having a window 4 on the insidesurface of which is a photo-cathode 6. Arranged opposite thephoto-cathode is an accelerating electrode 8 and a target supportelectrode 10 on which is mounted a suitable planar chargestoring target12 and an associated mesh collector electrode 14. As is well known inthe art the face of the target 12 remote from the photo-cathode 6 isarranged to be scanned by an electron beam 16 generated in the electrongun portion of the. tube (not shown).

Disposed in spaced-confronting relation with target 12 and a planeparallel to the target in the path of the scanning electron beam is aforaminate decelerating fieldforming, or field mesh, electrode 20carried by an annular support 22. The field mesh 20 may have a suitablepotential of, for example, approximately +200 volts positive withrespect to the cathode of the electron gun, which potential may beapplied through a conductive finger 24 engageable with the cylindricalconductive coating 26 which forms a focus electrode ,for electron beam16. After passing through the field mesh electrode 20 the electrons ofthe electron beam are decelerated by the electrostatic field between thefield mesh 20 and the target 12, which may have a potential of, forexample, +2 volts relative to the electron gun cathode, and hence theelectrons of the beam 16 arrive at the surface of the target with a verylow velocity of the order of a few volts. An additional annularelectrode 28', which may have a variable potential, facilitatesadjustment of the peripheral portion of the decelerating field.

After the scanning beam has performed the desired charge neutralizationat the target surface, the remaining electrons of the beam arereaccelerated in the reverse direction, i.e., to the right as shown at32 in FIGURE 1, and returned through the field mesh electrode 20 to theelectron multiplier portion of the tube (not shown). Some of thesecondary electrons emitted from the field mesh electrode 20 as a resultof impingement of the scanning electron beam thereon are likewiseaccelerated toward the electron multiplier portion of the tube andtherein serve as a source of spurious signals.

In accordance with my invention I minimize the generation of secondaryelectrons at the field mesh electrode by coating the field meshelectrode with a material having minimum secondary electron emissioncharacteristics, particularly in response to impingement of primaryelectrons having acceleration of about that provided by the potential ofelectrode 20, e.g., +200 volts. Such a coating is shown at 40 in FIGURE6, deposited on a mesh-like core or base metal 42, which may be for eX-ample copper or silver having 500-1000 openings per inch. One suchcoating material is chromium, which may be coated onto the mesh by anysuitable method such as evaporation, sputtering, or electrolytictechniques. I have found that with a field mesh having a thickness ofthe order of 0.2 mil, a coating of chromium on the surfaces of the meshnormal to the scanning electron beam and having a thickness of the orderof a few hundred angstroms satisfactorily inhibits emission of secondaryelectrons.

Other coating materials may be used, such as manganese, titanium,vanadium, or alloys or intermetallic compounds of chromium, manganese,titanium or vanadium. Moreover it is within the contemplation of thepresent invention that, rather than have such a coating, the field meshelectrode 20 may be made entirely of material such as chromium,manganese, or one of the other members of the group above defined havingthe desired low secondary emission characteristics. However, as apracthe invention may be' carried out in various ways and.

may take various forms and embodiments other than those illustrativeembodiments heretofore described. Accordingly, it is to be understoodthat the scope of the invention is not limited by the details of theforegoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A cathode ray tube of the orthicon type comprising in combination acharge imagestoring target, means for forming a charge image on thetarget, means for scanning an electron beam over said target, and aconductive foraminate electrode disposed in confronting adjacent relationship with the target in the path of said electron beam, saidforaminate electrode having surfaces exposed to said electron beam whichinclude at least one metal taken from the group consisting of chromium,manganese, titanium, vanadium, and alloys and intermetallic compoundsthereof, said surfaces being further characterized by having a secondaryelectron emission ratio of less than 1.0 upon impingement by primaryelectrons having energies of approximately 200 volts.

2. A cathode ray tube of the orthicon type comprising in combination acharge image storingtarg'et, means for forming a charge image on thetarget, means for scanning an electron beam over said target, and aforaminate electrode disposed in confronting adjacent relationship withthe target in the path of the electron beam, said foramion the surfacesof said base material exposed to said electron beam including a metaltaken from the group consisting of chromium, manganese, titanium,vanadium, and alloys and intermetallic compounds thereof, said coatinghaving a secondary electron emission ratio of less than 1.0 uponimpingement by primary'electrons having energies of approximately 200volts, said foraminate base layer being further characterized by beingof a good elec- 1954, p. as,

trically conductive metal having openings in the'rang'e of from about500 to about 1000 per inch and being about 0.2 mil thickness, saidcoating being at least about 300 angstroms thickness. f

3. A cathode ray tube :of the orthicon type having a planar charge imagestoring target, means including a photo-cathode for forming a chargeimage on one surface of the target, electron beam generating meansspaced from the opposite surface of the target, a focusing-electrodesurrounding said beam, and means for scanning said electron beam oversaid opposite surface of said target, a conductive mesh electrodedisposed in confronting adjacent relationship with said opposite surfaceof the target in the path of said electron beam, electrical connectormeans connecting said mesh to said focusingelectrode, means for applyingto said mesh electrode positive voltage relative to said target, toproduce an electron decelerating field from said mesh electrode to'saidtarget, an annular control electrode surrounding said decelerating fieldfor peripheral control-thereof, said mesh electrode having surfacesexposed to said electron beam including at least one metal taken fromthe group consisting of chromium, manganese, titanium, vanadium, andalloys and intermetallic compounds thereof. r 1

4. The invention as recited in claim 3 wherein said conducting meshelectrode includes a base metal mesh having a coating of chromiumdeposited thereon on the order of about 300 angstroms thickness.

References Cited in the file of this patent 'UNITED STATES PATENTS1,872,359 Sutherlin Aug. 16, 1932 2,108,132 Loro Feb. 15, 1938 2,846,609Espersen Aug. 5, 1958 3,086,138 Hendry Apr. 16, 1963 i FOREIGNPATENTS485,921 Canada -Q. Aug. 19,1952

7 OTHER REFERENCES I Bruining: Physics and Applications of SecondaryElectron Emission, McGraw-Hill Book Co., Inc., New York,

1. A CATHODE RAY TUBE OF THE ORTHICON TYPE COMPRISING IN COMBINATION ACHARGE IMAGE STORING TARGET, MEANS FOR FORMING A CHARGE IMAGE ON THETARGET, MEANS FOR SCANNING AN ELECTRON BEAM OVER SAID TARGET, AND ACONDUCTIVE FORAMINATE ELECTRODE DISPOSED IN CONFRONTING ADJACENTRELATIONSHIP WITH THE TARGET IN THE PATH OF SAID ELECTRON BEAM, SAIDFORAMINATE ELECTRODE HAVING SURFACES EXPOSED TO SAID ELECTRON BEAM WHICHINCLUDE AT LEAST ONE METAL TAKEN FROM THE GROUP CONSISTING OF CHROMIUM,MANGANESE, TITANIUM, VANADIUM, AND ALLOYS AND INTERMETALLIC COMPOUNDSTHEREOF, SAID SURFACES BEING FURTHER CHARACTERIZED BY HAVING A SECONDARYELECTRON EMISSION RATION OF LESS THAN 1.0 UPON IMPINGEMENT BY PRIMARYELECTRONS HAVING ENERGIES OF APPROXIMATELY 200 VOLTS.